Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: How to compute torque?

  1. Mar 8, 2010 #1
    Hello, i am confused in which equation to use when calulating torque....

    my textbook equation for torque is T = FD sin theta

    whilst the equation i have been taught is T = nBIA cos theta

    and... my textbook says that torque will be the greatest when applied at right angles (90 degrees). However the cos theta eqn. if you sub cos 90 it equals 0. Hence no torque

    what to do?

  2. jcsd
  3. Mar 8, 2010 #2


    User Avatar
    Homework Helper

    Re: Torque

    First torque is due to a mechanical force. Here torque is maximum when the force is perpendicular to the displacement.
    Second one the torque on a current carrying coil in a magnetic field. Here θ is the angle between the plane of the coil and the magnetic field.
  4. Mar 8, 2010 #3
    yea but for the second equation,when u sub θ =90 torque would equal zero, however when you sub zero you get a value. But θ = 0 means the coil is parallel with mag field hence no torque :S
  5. Mar 8, 2010 #4


    User Avatar
    Science Advisor
    Homework Helper

    Hello doc.madani! :smile:

    (have a tau: τ and a theta: θ :wink:)

    The θ in τ = nBIAcosθ is the angle between the direction of the field and the plane of the circuit.

    The θ in τ = FDsinθ is the angle between the direction of the two forces (in the torque) and the displacement between the two forces.

    The plane of the coil will contain a direction M perpendicular to the field, B. Let L be the direction perpendicular to M in the plane. We can assume that the coil is rectangular, with sides parallel to L and M.

    The displacement between the M sides is parallel to L, and the direction of the force on those sides is also parallel to L, so the torque is zero.

    The displacement between the L sides is parallel to M, and the direction of the force on those sides is perpendicular to B, in the plane of M and B, so the torque is proportional to the sin of the angle between M and the line "perpendicular to B, in the plane of B and M" …

    in other words, the cos of the angle between M and B. :smile:
  6. Mar 8, 2010 #5
    Re: Torque

    Thankyou Tiny-Tim very helpful :D
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook